A mathematical model of the mammalian distal nephron will be developed, comprised of cellular models of ascending Henle limb, distal tubule, and collecting duct. The model will represent sodium, potassium, and acid/base transport under normal and pathological conditions, and will predict renal excretion from distal delivery. The project begins with models of the three collecting duct segments; it will require development of two distal tubule segments plus an ascending limb, and then concatenation of all segments into a distal nephron. The segmental models will incorporate representations of specific membrane transporters: in distal tubule, the Na-CI cotransporter, and in ascending limb, the luminal Na-K-2CI and peritubular K-CI cotransporters. Segment-specific issues, as well as segmental interactions will be considered. For the collecting duct, proposed lesions underlying distal renal tubular acidosis (ATPase failure, base-exit defects, or paracellular leak) will be examined, and clinical tests for identifying these lesions will be simulated. In this, the objective is to examine the rationalization for the clinical taxonomy of distal tubular acidosis. The distal tubule model will be used to examine flow- dependence of potassium secretion, to estimate the component attributable to luminal gradient attenuation. This will be preliminary to quantifying the alkalinizing and potassium-wasting effect of thiazide diuretics, which act on distal tubule. In the ascending limb, an important focus will be identifying the modulated transporters responsible for cellular homeostasis, specifically, mechanisms used to accommodate large reabsorptive fluxes of sodium and ammonium, while preserving cell volume and pH. In ascending limb, the three different transport defects which all present as Bartter's syndrome will be simulated, to understand the potassium depletion alkalosis common to all three. The full distal nephron model will be required to critically examine the proposal that medullary interstitial potassium concentration modulates overall renal potassium and acid excretion: namely, that by blunting ascending limb sodium reabsorption, peritubular potassium sends more sodium to distal tubule and collecting duct where potassium secretion and base reabsorption depend on sodium delivery.